Systems are known where sensors are attached to structures, for example, such as bridges and buildings, to sense physical quantities such as vibrations in the structures, thereby monitoring the conditions of the structures, and thus monitoring generated deterioration and/or breakage of the structures. In such systems, it is not desirable to lay cables or the like for the power supply to the sensors and the acquisition of signals sensed by the sensors, because the space and laying operation therefor are required.
Therefore, systems referred to as “sensor networks” have been developed where multiple sensors that have wireless communication functions (also referred to as “wireless terminals with sensors” and “sensor nodes”) are adapted to work in a coordinated manner, thereby monitoring the conditions of structures. The individual sensor nodes are each composed of a sensor, a wireless chip, a microprocessor, and a power source (for example, a battery).
The sensor nodes are required to have high environment resistance, because it takes long to monitor the conditions of structures with the systems as mentioned above. Therefore, for the purpose of improving the environment resistance and/or insulation properties of sensor nodes, for example, as shown in FIG. 1, it has been proposed that constituent components (11 to 13) for a sensor node as described above are housed within a package member (20) formed of a ceramic material, and a lid member (30) formed of a ceramic material is bonded integrally to the package member (20) (for example, see Patent Document 1).
In the package member (20), conductor patterns may be buried, for example, such as a wiring (41) for electrically connecting the constituent components (11 to 13) for a sensor node, housed within the member, to each other, an antenna (42) for wireless communications, and an external electrode (43). The sensor node also desirably has a resistive loss reduced, and high-frequency characteristics improved. Therefore, the conductor patterns mentioned above are desirably formed of good conductors (for example, silver (Ag) and copper (Cu)). Because of the low melting points of the good conductors, it is common to use a low-temperature co-fired ceramic (LTCC: Low Temperature Co-fired Ceramics) as a material that forms the package member.
On the other hand, it is common to use, as a material that forms the lid member, alumina (Al2O3) which is relatively inexpensive and widely used, rather than the LTCC which is expensive as compared with common ceramic materials. In addition, alumina also has the advantages of favorable wettability to glass for use as a material (bonding material) for bonding the package member and the lid member, and also of high mechanical strength.